Il2 is an important lymphokine in the primary development of the immune response that promotes T-cell growth and proliferation. The structure of the complex of Il2 with Il2 receptor will be invaluable in understanding how Il2 mediates its important biological activity in the immune response. There are no structures of any T-cell receptor molecule determined yet, and a structure solution would represent a major advance in immunology in understanding a fundamental and key aspect of antigen recognition resulting in the cellular response that combats microbial infection in man.